In recent years, research and development have increasingly been conducted on electrophotographic photoreceptors having a photosensitive layer comprising a variety of organic photoconductive compounds as the main component. Particularly, a number of investigations have been carried out on the function-separating type photoreceptors in which a charge generation function and a charge transport function are shared with different materials, respectively, because each of the materials may be selected from a wide range of materials, and photoreceptors having optional performance can rather readily be prepared and many of them have been put into practical use.
These photoreceptors are manufactured by coating, on an electrically conductive support in the layered structure, compounds having the charge generating function and the charge transport function. Accordingly, the photoreceptor has generally a multilayer structure, and a coating composition, prepared by dissolving solid compounds in a solvent, is coated in the presence of the solvent.
Many organic solvents, having high dissolving power and an appropriate boiling point, particularly suitable for industrial use are, harmful. Solvents having excellent properties such as, for example, halogen based solvents exhibit a more adverse trend.
Solvents are employed for the production of the photoreceptors. Therefore, they cannot be employed if they possess neither good dissolving power nor appropriate boiling point, or if they adversely affect characteristics required for the electrophotographic photoreceptor. Thus, a solvent which meets all these requirements is urgently needed.
Titanylphthalocyanine having a maximum peak at 27.2.degree..+-.0.2.degree. of Bragg 2.theta. for Cu--K.alpha. line contains several water molecules per titanylphthalocyanine molecule in the crystal. These water molecules assist the dissociation process of the ion pair formed from the excited state of S1 in the course of the generation of a carrier to a free carrier with borrowing an electric field force. Accordingly, it has been known that in this crystal, a high quantum yield is obtained which is not found in the other crystal form. (Kinoshita, Summaries of Electrophotographic Society Third Technical Course, page 22, 1991) Therefore, in order to consistently take out the characteristics of titanylphthalocyanine having a maximum peak at 27.2.degree..+-.0.2.degree. of Bragg 2.theta. for Cu--K.alpha. line, it is important that water molecules are capable of being abundantly present in the crystal. Due to this, there has been a problem of great variation in sensitivity under environments of low temperature and humidity, which reduce water content in the photoreceptor upon drying. In order to solve this problem, conventionally, a moisture maintaining technique has been investigated in which diol is incorporated in a carrier generating layer. However, when a compound comprising an OH group having high polarity, such as diol, is abundantly present in the photosensitive layer, there have been associated problems in which variation in electric potential is large during usage and no sufficient durability is obtained.